The present invention relates to a process for the racemate resolution of 2-hydroxypropionic acids by reacting the racemic acid with an optically active base and subsequently separating off a diastereomeric salt of acid and base. 2-hydroxypropionic acids are important intermediates for the synthesis of plant protection agents and pharmaceuticals. Since the action of such substances is frequently due to only one enantiomer, it is desirable to prepare these active ingredients in high optical purity. It is therefore advisable to plan the synthesis in such a manner that racemic intermediates can be separated into their enantiomers in order subsequently to obtain enantiomerically pure end products.
WO 93/13075, which is equivalent to U.S. Pat. No. 5,547,972, describes propionic acids 2-substituted by heterocycles as potent endothelin-receptor antagonists for treating cardiovascular disorders. These active ingredients are synthesized using 2-hydroxypropionic acids as intermediates.
WO 96/11914 describes the preparation on a laboratory scale of (S)-2-hydroxy-3-methoxy-3,3-diphenylpropionic acid by racemate resolution using L-proline methyl ester (Example 10) and (S)-1-(4-nitrophenyl)ethylamine (Example 11). In this manner a yield of 35% based on the racemate and an optical purity of 99.8% are achieved.
However, it has been found that when this described reaction step was scaled up (several kg to 100 kg), additional working steps became necessary in order to ensure a high optical purity. The diastereomeric salt of (S)-2-hydroxypropionic acid and (S)-1-(4-nitrophenyl)ethylamine crystallizes with difficulty and can therefore not be filtered off readily, either, so that some of the mother liquor remains in the crystals together with the enantiomer to be separated off. Only when the crystals were additionally stirred in the tank together with fresh solvent, and when the crystals which had been filtered off once more had been copiously rewashed, was the required optical purity obtained.
It is an object of the present invention, therefore, to provide a process for racemate resolution which does not have the abovementioned disadvantages but can readily be carried out on an industrial scale.
We have found that this object is achieved by a process for racemate resolution of 2-hydroxypropionic acids by reacting the racemic acid with an optically active base and subsequently separating off a diastereomeric salt of acid and base, which comprises using 1-(4-chlorophenyl)ethylamine as optically active base.
The preparation of 2-hydroxypropionic acids is familiar to those skilled in the art from organic synthesis manuals as described, for example, in WO 96/11914.
The process according to the invention is particularly suitable for the racemate resolution of 2-hydroxypropionic acids which are monosubstituted or polysubstituted at the 3 position, in particular 2-hydroxypropionic acids which, at the 3 position, bear from one to two aryl radicals, preferably phenyl.
A particularly preferred embodiment of the process according to the invention is the racemate resolution of 2-hydroxy-3-methoxy-3,3-diphenylpropionic acid and 2-hydroxy-3-methyl-3,3-diphenylpropionic acid.
The process according to the invention is usually carried out in such a manner that a solution of xc3x97mol of the racemic 2-hydroxypropionic acid, if necessary with heating, is admixed with about 0.5-0.6xc3x97mol of optically active 1-(4-chlorophenyl)ethylamine and then one of the diastereomeric salts formed is separated off.
In a preferred embodiment, seed crystals of the diastereomeric salt to be separated off are added to the solution and the solution is allowed to cool slowly, which produces a good crystallization. The seed crystals can be added after all of the optically active base has been added or even earlier, if only a portion, preferably about half, of the required base has been added.
If the other enantiomer of the 2-hydroxypropionic acid is wanted, this can also then be isolated from the mother liquor.
A preferred solvent for the racemate separation is a mixture of alcohol and ether, in particular of methanol and methyl tert-butyl ether (MTB). An alcohol, in particular isopropanol, is preferably used for the racemate resolution of 3-hydroxy-3-methyl-3,3-diphenylpropionic acid.
The process according to the invention has the advantages that it produces enantiomerically pure 2-hydroxypropionic acid in a very good yield and can be scaled up to the industrial level without problems. The crystals of the diasteromeric salt of 2-hydroxypropionic acid and 1-(4-chlorophenyl)ethylamine settle surprisingly well in the mother liquor and can therefore be separated off by filtration without problems. Complex rewashing of the crystals to achieve the desired enantiomeric purity is not necessary. This is a substantial advantage in comparison with the diastereomeric salt of 2-hydroxypropionic acid and 1-(4-nitrophenyl)ethylamine. Thus, complex transfer operations, stirring steps and rewashing steps can be saved, which decreases energy consumption and the amount of solvent, and the space-time yield of a given plant can be substantially increased.